Electrical devices need electrical energy supplied by power modules to operate. Some electrical devices have to consume more electrical energy. Thus, more than one power module must be provided to those electrical devices which consume more electrical energy. However, in practice, each power module has different rate of power consumption. For example, two power modules are connected in parallel for supplying electrical energy to an electrical device. After a period of time, the two power modules retain different electric quantities. For example, one retains 80% electric quantity and the other one retains 70% electric quantity. When power modules retain different electric quantities, one of them will run out first. Therefore, the total output current can not meet the demand so that the total output of the power modules can not be optimized.